Turret and Capstan lathes are the product of the natural evolution of engine lathe. In the field of mass production centre lathe, though, very versatile machine tool, is not suitable for manufacture of parts on repetitive basis.
Turret and capstan lathes bridge up the gap between the manual engine lathe and the automatic lathe and are most practical for short production runs as well as continuous ones. The chief distinguishing feature of this type of lathe is the multiple tool holders which enable the setting up of all the tools for a job; except for sharpening they need no further handling.
Although considerable skill is required to set and adjust the tools properly, but once they are set, little skill is required to operate them and may be operated by semi-skilled operators. Furthermore, many parts can be produced before any new adjustment is made. Eliminating the set up time between operations reduces the production time tremendously. The development of this group of lathes has made interchangeable manufacture possible.
Multiple tooling is provided by replacing the simple tail stock with a rotating or indexing hexagonal turret or capstan head on which several tools can be mounted and preset according to the requirements of various operations to be done on the job.
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The tool post is replaced by a square cross-slide turret. Sometimes cross-slide may also support a square turret on the rear, and thus making a total of 14 tools which can be mounted at a time. The movement of these tools is controlled by adjustable stops.
Turret and Capstan lathes give quite effectively the normal accuracy needed for interchangeable mass production. A standard machine gives accuracy of 0.002 mm for turning and boring. Capstan lathes are used mainly for bar work and turret lathes are used for larger work in the form of castings and forgings.
Fig. 32.1 shows the general arrangement for turret and capstan lathes. In the case of turret lathe, the turret is mounted directly onto the auxiliary slide but in the case of capstan lathe, the turret, instead of being carried on auxiliary slide as in the case of turret lathe, is carried on a separate slide (ram) which can be moved back and forth by a star wheel.
When moved back, it will index the turret to the next tool station. This slide, together with the turret, is attached to an auxiliary slide which is locked to the bed after the machine has been set up. Since no separate slide is involved in case of turret lathe, it becomes a rigid construction making it suitable for heavy work.
Since change of speed should be effected in shortest possible time, this is achieved on capstan and turret lathes by using a constant mesh, all-geared head stock; change of speed being performed by operating multiple lathe-friction clutches.